Large heavy duty nozzles are typically used to project streams of pulverized coal and air into the fireball area of a combustion chamber in, for example, an electrical generating plant. Four nozzles may be located at respective corners of the combustion chamber to aim the pulverized coal/airstreams tangentially of the fireball. The nozzle tips are typically hingedly connected to the main conduit portion of the nozzle such that the fireball may be raised and lowered within the combustion chamber for temperature control purposes.
The nozzle tip is exposed to an extremely harsh environment in actual operation; i.e., pulverized coal is highly abrasive and the flow thereof through the nozzle rapidly wears portions of the nozzle away. In addition the heat created by the fireball and the high temperature gradients produced by the through flow of air subject the nozzle structure to extreme internal stresses caused by large temperature differentials.
The above-described factors create the need for nozzle tip structures which are more capable of withstanding the harsh environment and which are relatively easily replaced with the minimum of down time, preferably from inside the combustion chamber.
The prior art includes U.S. Pat. No. 4,672,900 issued Jun. 16, 1987 to Richard Santalla et al which illustrates the four cornered tangential firing arrangement described above. With more specific relevance to nozzle tip structures, the prior art further includes U.S. Pat. No. 520,739 issued Jun. 4, 1985 to Michael McCartney et al. disclosing a three-piece nozzle tip comprising a box-like base portion, a box-like temperature resistant end cap and an abrasion resistant cast insert which is adapted to be sandwiched between the base portion and the end cap. The three-piece unit is held together by way of connector bolts. The end cap and the cast insert are individually replaceable.
This unit, although complex, represents a theoretical advantage over the more standard one-piece nozzle tip in which inner and outer rectangular housings are held together in essentially concentric spaced relationship by means of welded ribs. The space between the inner and outer housings defines an air flow path while the interior of the inner housing defines the flow path for the pulverized coal. The inner housing is typically provided with a plurality of parallel baffles hereinafter called "splitter plates" which ensure that the coal is adequately divided as it enters the combustion chamber. This unit is hingedly attached to the outlet end of the nozzle conduit for vertical aiming adjustment as previously described.
Practical experience shows that the one-piece nozzle tip is highly subject to abrasion wear, particularly at the junctions between the splitter plates and the sidewalls of the inner housing. In addition these structures typically exhibit a number of cracks and fractures in the outer housing, the inner housing and the ribs between the housings due to the aforementioned stresses caused by high temperature gradients.
Another prior art patent, U.S. Pat. No. 4,356,975 issued Nov. 2, 1982 to Roman Chadshay, also deals with the problems of rapid wear in the nozzle tip due to the abrasive affects of pulverized coal and the exposure of the nozzle tip to high temperatures. The disclosure of this patent proposes to solve these problems by making the splitter plates in two physically different parts and assembling these parts in the inner shell of the nozzle; i.e., the two portions of each of the splitter plates are arranged in serially abutting relationship such that the edge of one plate portion lies against the edge of another plate portion. The splitter plate portions are aligned by means of rail like members.